So first thing is that didn't fail in the past and another thing is why
does it fail at all?

I only have
PreReq: myspell-dictionary
in a sub-package and
%triggerin -- myspell-dictionary
%triggerpostun -- myspell-dictionary

ah, interesting ...
well, in order to be able to test-install a package after build (we already
do this in the internal systems), we had to add the PreReq(ed) packages
to the build-environment. As this was not done before, the expansion error
did never show up.
This means you are building in a project/repository where no package satisfies
this dependency. However no idea why that is happening ...

That certain PreReq was a bit bogus anyway (aka not really needed IIRC).
I have another package where it causes an expansion error where I think
OBS is on fault.